1. Field of the Invention
The present invention relates to a mobile communication system with reduced inter-cell interference (ICI) and a method thereof, and more particularly, to a mobile communication system with reduced ICI by controlling an uplink signal's transmission signal power based on power measurement of a signal received from a base station, and a method thereof.
2. Discussion of the Background
Mobile communication technology includes a mobile communication device usable by a user in a moving object, such as a train, a ship, or an airplane, or while the user is walking. FIG. 1 illustrates a cell structure of a mobile communication system in which a mobile communication device may be used according to the conventional art.
As shown in FIG. 1, a mobile communication system includes base stations 111, 112, and 113, and mobile communication devices, which may be referred to as mobile stations or a mobile communication terminals (hereinafter, referred to individually as a “terminal”) 121, 122, and 123. Each base station 111, 112, and 113 provides mobile communication services to a respective wireless communication area called a “cell.” Each terminal 121, 122, and 123 may be located in a cell corresponding to one of the base stations 111, 112, and 113, and respectively receives mobile communication service via the base stations 111, 112, and 113.
In this instance, a base station 111, 112, or 113 in a cell may be affected by multiple access interference corresponding to signal interference from a terminal located within the cell, and inter-cell interference (ICI) corresponding to signal interference from a terminal located in a neighboring cell.
Orthogonal frequency division multiplexing (OFDM) technology has been developed, and may be able to reduce multiple access interference. However, ICI, and in particular, ICI of an uplink channel has not been solved through the use of OFDM.
In many mobile communication systems, a terminal near a cell boundary may have signal distortion due to the ICI. Accordingly, for reliable data transmission, channel coding is performed with an extremely low channel coding rate and then data is transmitted. For example, a portable internet Wireless Broadband (WiBro) standard uses a coding rate of 1/12.
Various solutions have been proposed for reducing the ICI problem. FIG. 2 illustrates an example of a frequency band allocation method based on fractional frequency reuse (FFR) according to the conventional art.
Referring to FIG. 2, terminals located in the centers of cell (1) 221 (cell 221), cell (2) 222 (cell 222), and cell (3) 223 (cell 223), use the same frequency band 210. However, a terminal near a cell boundary may not use one predetermined frequency band from among three frequency bands 211, 212, and 213 or may use the predetermined frequency band at a lower power to avoid frequency duplication with a neighboring cell. For example, a terminal near a boundary of the cell 221 may not use a first fractional frequency band 211 or may use the first fractional frequency band 211 with a low power. Also, another terminal near to a boundary of the cell 222 may not use a second fractional frequency band 212 or may use the second fractional frequency band 212 with a low power. Also, still another terminal near to a boundary of the cell 223 may not use a third fractional frequency band 213 or may use the third fractional frequency band 213 with a low power. As a result, the terminal may reduce ICI, but a frequency reuse factor of a terminal located in an outer boundary of a cell is reduced to ⅔ without regard for the actual use of the three frequency bands prior to avoiding frequency duplication with a neighboring cell.
FIG. 3 illustrates another frequency band allocation method for ICI reduction according to the conventional art. Referring to FIG. 3, a cell is divided into a central area (a white area) and an outer boundary (a shaded area). In this instance, frequency bands are allocated so that a mobile terminal in the central area may use a common frequency band with neighboring cells, and a terminal of the outer boundary may use a frequency band that is not used in neighboring cells.
Specifically, a cell (2) 302 (cell 302), a cell (3) 303 (cell 303), a cell (4) 304 (cell 304), a cell (5) 305 (cell 305), a cell (6) 306 (cell 306), and a cell (7) 307 (cell 307) neighbor a cell (1) 301 (cell 301). A first frequency band is allocated to an outer boundary of the cell 301, but is not duplicated with a second frequency band or a third frequency band allocated to outer boundaries of the cells 302 through 307. An outer boundary area of the first frequency band is allocated is marked in black. Also, the cells 302, 304, and 306, which are allocated with the second frequency band and for which the outer boundaries are hatched with dots, are spaced apart from each other. Also, the cells 303, 305, and 307, which are allocated with the third frequency band and for which the outer boundaries are hatched with diagonal lines, are spaced apart from each other. Specifically, the ICI reduction scheme shown in FIG. 3 may allocate a frequency band not to be used between neighboring cells in an outer boundary with the most severe ICI, and thereby reducing the ICI.
In addition to the ICI reduction schemes described with reference to FIG. 2 and FIG. 3, other various types of ICI reduction schemes have also been proposed. The ICI reduction schemes are commonly based on an idea of ICI coordination/avoidance that limits frequency use time or frequency resource for a terminal located in an outer boundary of a cell.
However, the ICI reduction schemes based on the ICI coordination/avoidance, including the FFR scheme, have many problems.
First, in practice, a cell area has a distorted shape that is different than a theoretical hexagonal cell arrangement. Accordingly, it is more difficult to separately manage frequency bands for the central area and the outer boundary.
Second, since an available frequency band is reduced, trunking efficiency may be reduced. Specifically, wireless resources may be exhausted when more terminals are located in the outer boundary of the cell.
Third, in comparison to when the same entire frequency band is used in all cells, frequency hopping may be reduced. Accordingly, frequency diversity effect may be reduced, and thus a multi-path signal may not be effectively processed.
Fourth, since a frequency band is allocated to a terminal located in an outer boundary of a cell based on a relation with a neighboring cell, flexible cell planning is difficult. For example, adding an additional base station and an additional cell between existing cells, a new frequency band should be allocated to cells adjacent to the additional cell, and this may require the modification of the cell planning.
Fifth, a portion of the frequency band may be unused even when there is no ICI. Accordingly, wireless resources may not be effectively managed.
Finally, in the conventional ICI reduction schemes as described above, an upper layer service control point (SCP) or a mobile switching center (MSC) should be in charge of cell planning and coordination for a plurality of base stations. However, this is inconsistent with ALL-Internet Protocol (IP), which is of the trend for the next generation communication network.
Accordingly, there is a need for a new technology in which a mobile communication system can adaptively manage uplink resources to reduce ICI.